Sensitive hydrophone

ABSTRACT

An optically interrogated acoustic ASW surveillance system is disclosed in which a laser beam is used to illuminate floating acoustic transducers. These transducers have beam-splitting optics and a submerged monocrystalline diaphragm which modulates one portion of the split beam in response to acoustic signals arriving through the water. When this portion of the beam is colinearly recombined with the unmodulated portion of the split beam, an amplitude-modulated light signal is produced which is directed back to and detected at the laser source.

United States Patent [72] Inventor Max N. Yoder Washington, D.C. [2|Appl; No. 820,405 [22] Filed Apr. 30, 1969 [45] Patented Oct. 5, 1971 1I73 Assignee The United States of America as represented by theSecretary of the Navy [541 SENSITIVE HYDROPHONE 5 Claims, 1 Drawing Fig.

[52] US. C

250/ 199. 356/5 [51] int. n colv 1/00 [50] Field oiSearch 181/.5 B;331/945; ISO/I99; 340/14 [56] Reierences Cited UNlTED STATES PATENTS 3,l 75.088 3/l965 Herriott 250/199 3,373,401 3/1968 Bayer 331/945 X OTHERREFERENCES Foster, A Laser Device for Remote Vibration Measuremerit,IEEE Transactions on AES, March i967. Vol. AES-3 02pp. 154-1571811513Honig, Laser Benioff Strain Seismometer, Proc. IEEE; Vol. 53,No. 1331-945 Primary Examiner-Rodney D. Bennett. Jr. AssistantExaminer-Joseph G. Baxter Attorneys-R. l. Tompkins, L. I. Shrago and R.K. Tendler ABSTRACT: An optically interrogated acoustic ASW surveillancesystem is disclosed in which a laser beam is used to illuminate floatingacoustic transducers. These transducers have beam-splitting optics and asubmerged monocrystallinc diaphragm which modulates one portion of thesplit beam in response to acoustic signals arriving through the water.When this portion of the beam is colinearly recombined with theunmodulated portion of the split beam, an amplitude-modulated lightsignal is produced which is directed back to and detected at the lasersource.

i l I I l g was l l i i.., l 20 l I 30 l l PATENTED [JCT 5 I97! R w Z 27 W w I/ MI E R w 5 1K w W, & ||.Y m Z 1 l Wi 6 X D MK D06 a FDR;

I NVENTOR.

acoustic transducer which is sensitive to low-level subsurface acousticradiation in the ocean. The surveillance of large portions of the oceanby a multiplicity of acoustic detectors scattered over alarge area hassuffered primarily from the cost and complexity of the equipmentnecessary to provide acoustic information. Self-powered modules havebeen developed which provide radio signals indicating subsurfaceacoustic activity. These modules have a relatively short lifetime andcannot be deployed in great numbers because of their cost. in addition,these devices require operation from a relatively stable platform whichis difficult to achieve at sea. The present device is a low-cost,nonstabilized passive acoustic sensor which is illuminated with a laserbeam. It is possible to deploy hundreds of these lowcost devices over agiven area of the ocean and to sample these devices periodically byoverflying them. The overflights'may becarried out periodically byhelicopter, blimp or by slow-moving aircraft. On board this aircraft isa laser-transmitting system and a system which detects reradiated lightfrom the passive floating modules.

The module itself is a free-floating device which requires no internalsource of power. In its operation, monochromatic radiation from thelaser which impinges on this device is optically split. One portion ofthis beam is reflected back to the beam-splitting device by a fixedmirronThe other portion of the split beam is modulated by a crystallinediaphragm which has a side exposed to the aqueous medium. Movement ofthis diaphragm due to the arrival of subsurface acoustic energy variesthe path length for this portion of the split beam. This portion of thebeam is redirected back through'the beam-splitting device and iscombined oolinearly with the other portion of the beam to provide aphase-modulated return beam. This phase modulation produces an amplitudemodulation of the intensity of the beam returned and when detected by aphotodetector produces an amplitude-modulated signal. The modulation isdependent upon the flexure of the aforementioned diaphragm. Displacementof the diaphragm by only 5.3A. in response to a longitudinally polarizedsound signal modulates the return beam at a level of 5.3/5,300 with alaser frequency output wavelength of 5,300A. This represents 0.1 percentof the amplitude of the return beam or a 30-db modulation of the signal.Low frequency components of the return signal due to the heave, pitchand roll of the module are filtered from the output of the photodetectorby a conventional high-pass filter so that only acoustic energy ismeasured by the subject system.

lt is therefore an object of this invention to provide an opti callyinterrogated acoustic transducing system for use in the detection ofundersea propagated acoustic energy.

It is a further object of this invention to provide a sensitive acousticpassive floating transducer which is interrogated by a monochromaticbeam of light.

It is another object of this invention to provide a passive, low-costacoustic transducing module for deployment at spaced points on thesurface of the ocean.

lt is a still further object of this invention to provide asemisubmerged passive acoustic transducing unit having an opticalbeam-splitting device and means in contact with the surrounding aqueousmedium for modulating one of the split beams.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description thereof whenconsidered in conjunction with the accompanying drawing.

The single drawing represents a passive acoustic transducing system inwhich the transmitting and receiving unit is designated by the dottedbox 1 and transducing unit by housing 2. The transmitting unit which maybe conveniently carried within a helicopter or slow-moving aircraftcontains a CW laser 3 which in the preferred embodiment lases at 1.06microns and is frequency doubled to 0.53 micron. Beam 4 from laser 3which is passed through suitable focusing optics 5 is redirected by aplanar mirror 6 and againby another planar mirror 7 which forms aportion of the receiving optics. This monochromatic beam is directeddownwardly so as to scan across the surface of the ocean with themovement of the aircraft. When this beam impinges on wide-angle lens 8of any one of the floating modules, it is directed towards ahalf-silvered mirror 9 within housing 2. This mirror splits the incomingbeam and a portion 19 is directed at right angles towards one of thewalls of the housing. This beam impinges upon a planar mirror 11 affixedto this wall and is redirected back along the same path to the haltsilvered mirror. The redirected and split portion of the beam is thenreflected back colinearly along the axis of the incoming beam throughlens 8. The other portion of the split beam continues along the incomingaxis through half-silvered mirror 9 and impinges upon a monocrystallinediaphragm 12. This diaphragm is mounted at an orifice defined by walls13 of the housing such that the lower face of this diaphragm is exposedto sea water diagrammatically shown at 14 to surround a portion ofhousing 2. The aforementioned beam is reflected back by the top surfaceof the crystalline material. This crystalline material may be coated soas to be optically reflective at the wavelength of the monochromaticlight used. The crystalline material when highly polished may also serveas a reflector for this light. Incoming acoustic energy is focused ontothe lower side of this diaphragm by cone 15 so as to maximize the effectof acoustic radiation on the diaphragm.

The monocrystalline diaphragm is made as thin as possible while stillretaining a maximum of rigidity or Rockwell hardness. This crystallinediaphragm in one embodiment may be either silicon or quartz. Because ofthe extremely hard nature of these materials, the diaphragm in thisconfiguration is affixed to housing 2 with a soft resilient material(not shown) interspersed between the upper edge of the diaphragm andhousing lip 13. The size of the orifice in the housing is deter- I minedto be some fraction of the wavelength of the center frequency of thetransducer so as to produce a maximum amount of resonance of thediaphragm to the signals to be detected. It will be appreciated thatthis transducer is basically a low Q, broad-bandwidth system. its mainfunction is to provide an indication of subsurface acoustic activitywhich may contain a'broad spectrum of acoustic frequencies.

Moreover, it will be appreciated that while in the preferred embodimentthis transducing device functions as an indicator of the presence ofacoustic signals it is possible to extract spectral information from thereturn beam. Acoustic impedance transformers can be placed on the bottomof the diaphragm to reduce the acoustic bandwidth of the acoustictransducer and optimize the response of the device for acoustic signalsof a predetermined frequency range.

Housing 2 is kept in an upright position by enclosed weights 16. Theunitary construction shown in the diagram is preferable to maintain thebuoyancy of the entire structure. Weights 16 may be positioned withinthe housing both to act as a selfrighting device and for acousticdamping of the lower surface of the module.

The wide-angle lens used with this module is necessary so that laserbeams which are not directly above the module can be modulated. The useof this wide-angle lens thus precludes the necessity for stabilizing theentire module. Since the optical paths between the half-silvered mirrorand both the fixed mirror 11 and the diaphragm 12 are relativelyconstant and of approximately the same length, sea motions will noteffect the phase-modulated signal returned by the module. Since thereflected beam from the module is returned colinear with the transmittedbeam, the exact position of the module can be determined if thedirection of the transmitted beam is known. This return beam is shown asdotted line 20. Because of the colinearity of the two components of thisreturn beam, amplitude modulation occurs through optical mixing. Thereturn beam is shown entering focusing optics 21. These optics focus thebeam onto a conventional photodetector 22 which acts as a summationdevice. Since the electric field polarity for both the components of thereturn beam are in the same direction. a phase cancellation takes placewhich creates an effective amplitude modulation of the return beam atthe photodetector. The output of the photodetector is coupled to aconventional AM detector 23. The output of the AM detector is filteredby a conventional high-pass filter 24 and coupled to a display device.25, which in one embodiment is a strip chart recorder. This recorder iscoupled to a dead reckoning track computer 26 which feeds the displaydevice with information concerning the X and Y coordinates of thehelicopter or aircraft. This plot is displayed conventionally as the DRTof the aircraft. Superimposed on this dead reckoning track is anamplitude-modulated signal from the particular module which is beingoverflown whenever the module detects acoustic subsurface radiation.Depending on the number of modules deployed at any given time, accuratelocation of the subsurface acoustic force may then be read off directlyfrom the display.

The present system thus provides unusual sensitivity to hydroacousticradiation because of the phase-modulated split beam optics and themonocrystalline diaphragm combination. The phase-modulated signal isconveniently converted to amplitude modulation which not only maintainsthis sensitivity but allows processing by simple inexpensive AMequipment. This system has the further significant advantage that themultiplicity of hydrophones deployed do not require an internal powersupply and as such have a relatively long lifetime.

What is claimed is:

1. Apparatus for detecting acoustic signals propagating beneath thesurface of a fluid medium comprising:

a housing adapted to float at the surface of said fluid median acousticsignal detector positioned in said housing with its sensing element incommunication with said fluid medium;

an optical system accommodated within said housing and adapted to beilluminated with a laser beam,

said optical system including means for directing a portion of saidlaser beam over a first optical path of fixed length and thereafterredirecting said portion back along the path of said laser beam and fordirecting another portion of said beam over a second optical path whoselength varies in response to the amplitude of any acoustic signaldetected by said sensing element and thereafter redirecting saidlast-mentioned portion back along the path of said laser beam such thatboth redirected portions of said beam interact because of their colinearorientation to produce a modulated return beam whenever acoustic energyis detected by said sensing element.

which is above the free surface of said fluid medium, said v wide-anglelens when illuminated with a monochromatic beam of light originating ata remote laser focusing said beam on said light reflective surface; andoptical means accommodated within said housing for directin a rtion ofsaid beam over a path of a predetermined ixe length and for thereaftercombining said portion with the light beam reflected from saidreflecting surface, the resulting beam having an amplitude which variesin accordance with the detected acoustic signals.

this resulting beam thereafter proceeding through said wide-angle lenstoward a remote detecting location.

3. A system for detecting at a remote location acoustic signalspropagating beneath the surface of a fluid medium comprising, incombination,

a housing floating on the surface of said fluid medium;

an acoustic signal detector accommodated within said floating housingwith its sensing element adapted to receive acoustic signals present inthe surrounding fluid medium;

a laser located at said remote location and adapted to illuminate anexposed area of said housing with a monochromatic beam of light;

means within said housing for splitting said monochromatic beam of lightand for directing one portion thereof over a first optical path which isof a fixed length and for directing another portion thereof over asecond path whose length varies in accordance with the magnitude of anyacoustic signals detected by said signal acoustic detector,

said last-mentioned means also operating to thereafter recombine saidportions of said split beam and to redirect the beam resulting therefromback towards said remote location; and means at said remote location fordetecting any amplitude modulation of the returning beam thereby toascertain the nature of any acoustic signals propagating in said fluidmedium in the vicinity of said housing.

4. in an arrangement as defined in claim 3 wherein said acoustic signaldetector includes a thin, monocrystalline diaphragm with one planarsurface thereof reflective to light.

5. In an arrangement as defined in claim 3 wherein a wideangle lens ismounted on an exposed surface of said housing so as to focus anyincoming light into the interior of said housing.

1. Apparatus for detecting acoustic signals propagating beneath thesurface of a fluid medium comprising: a housing adapted to float at thesurface of said fluid mediUm; an acoustic signal detector positioned insaid housing with its sensing element in communication with said fluidmedium; an optical system accommodated within said housing and adaptedto be illuminated with a laser beam, said optical system including meansfor directing a portion of said laser beam over a first optical path offixed length and thereafter redirecting said portion back along the pathof said laser beam and for directing another portion of said beam over asecond optical path whose length varies in response to the amplitude ofany acoustic signal detected by said sensing element and thereafterredirecting said last-mentioned portion back along the path of saidlaser beam such that both redirected portions of said beam interactbecause of their colinear orientation to produce a modulated return beamwhenever acoustic energy is detected by said sensing element. 2.Apparatus for detecting acoustic signals present in a fluid mediumcomprising, in combination, a housing adapted to float at the freesurface of said fluid medium; an acoustic signal detector accommodatedwithin said housing with its sensing element adapted to receive acousticsignals present in the surrounding fluid medium, said sensing elementhaving one surface thereof which is light reflecting and which isdisplaced in accordance with the amplitude of said received acousticsignals; a wide-angle lens supported by said housing at a location whichis above the free surface of said fluid medium, said wide-angle lenswhen illuminated with a monochromatic beam of light originating at aremote laser focusing said beam on said light reflective surface; andoptical means accommodated within said housing for directing a portionof said beam over a path of a predetermined fixed length and forthereafter combining said portion with the light beam reflected fromsaid reflecting surface, the resulting beam having an amplitude whichvaries in accordance with the detected acoustic signals, this resultingbeam thereafter proceeding through said wide-angle lens toward a remotedetecting location.
 3. A system for detecting at a remote locationacoustic signals propagating beneath the surface of a fluid mediumcomprising, in combination, a housing floating on the surface of saidfluid medium; an acoustic signal detector accommodated within saidfloating housing with its sensing element adapted to receive acousticsignals present in the surrounding fluid medium; a laser located at saidremote location and adapted to illuminate an exposed area of saidhousing with a monochromatic beam of light; means within said housingfor splitting said monochromatic beam of light and for directing oneportion thereof over a first optical path which is of a fixed length andfor directing another portion thereof over a second path whose lengthvaries in accordance with the magnitude of any acoustic signals detectedby said signal acoustic detector, said last-mentioned means alsooperating to thereafter recombine said portions of said split beam andto redirect the beam resulting therefrom back towards said remotelocation; and means at said remote location for detecting any amplitudemodulation of the returning beam thereby to ascertain the nature of anyacoustic signals propagating in said fluid medium in the vicinity ofsaid housing.
 4. In an arrangement as defined in claim 3 wherein saidacoustic signal detector includes a thin, monocrystalline diaphragm withone planar surface thereof reflective to light.
 5. In an arrangement asdefined in claim 3 wherein a wide-angle lens is mounted on an exposedsurface of said housing so as to focus any incoming light into theinterior of said housing.